Post-transplantation lymphoproliferative disorders (PTLD) occur after solid organ and bone marrow transplantation. They are predominantly of B-cell and occasionally of T-cell lineage. We report a case of PTLD of natural killer (NK) cell lineage. A renal allograft recipient developed progressive pancytopenia 1 year after transplantation. Serial bone marrow biopsies showed an increasing infiltration by large granular lymphoid cells. A subsequent leukaemic phase also developed with systemic infiltration of other organs. Immunophenotyping showed that these cells were CD2+, CD3-, CD3epsilon+, CD56+, CD94+, CD158a- and CD158b-. In situ hybridization showed Epstein-Barr virus (EBV) infection of the neoplastic cells. Genotypical analysis showed the T-cell receptor gene in germline configuration and clonal EBV episomal integration. The overall features were consistent with NK cell lymphoma/leukaemia. The patient did not respond to cessation of immunosuppression or anti-EBV treatment. Combination chemotherapy was given, but the patient died ultimately of disseminated fungal infection. In conclusion, we have demonstrated that NK cell lymphoma is another rare type of PTLD that appears to be highly aggressive and therefore may require early chemotherapy to improve treatment outcome.